Rudimentary Behaviours

Biting Birds and Piloerection

Anybody who knows that the kiwi is foremost and for all a bird (and not a fruit) knows that this New Zealander has no wings – only a few rudimentary bones remain of what were once the winsgs of its ancestors. Whales have no hind limbs, so the entire pubic girdle became vestigial. Certain toes are often superfluous and consequently through the process of selection have diminished in size or disappeared completely as in the horse and other hoofed animals. Teeth, too, as with our so-called wisdom teeth can be vestigial and eye rudiments in cave organism are another example. The anatomical concept of rudimentary organs is therefore easily understood, but we could ask ourselves whether there might not also be something like a rudimentary behaviour or functionally useless action steeped in evolutionary history.

We could, for example, consider snails; snails with very small shells that do not offer much protection at all. Such snails still attempt to withdraw into their tiny shells, even though that is physically impossible. There are other examples: consider insect species with vestigial wings: adults of the silkworm moth Bombyx mori are a good example. They still perform flight movements, beating their short wings vigorously and yet it’s all in vain, for it won’t help them one little bit to lift off and fly about. Or ostriches: their energetic wing flapping leads to no more than some awkward little jumps. Or take the stump-tailed gecko Brookesia. This reptile attempts to use its club-shaped tail for grabbing and holding on to branches when climbing a tree – an impossible task. And have you observed how floppy-eared dogs try to prick their ears upright when wanting to hear a faint sound? The behaviour inherited through ancestral lines is there, but anatomical constraints will not allow the behaviour to functionally make sense.

The vast majority of birds, for instance, even the crane with its long and formidable beak, rather than stab will bite when cornered! Clearly, using the bill as a dagger would be more effective, but the reptilian ancestors of the birds had teeth and bit, and that’s why birds, even today, still bite (except for the heron, which is known to spear fish and frogs with its beak). Another relict behaviour that is traceable from reptilian ancestors right into mammalian species, is the scratching of the head. Most animals, using one of their hind limbs, lift the latter over the fore-limb, even if this results in a clumsy action as in birds where the wing has to be lowered in order for the leg to pass over it.

In humans piloerection, the phenomenon of developing goose pimples, is a rudimentary response going back to the times when our hairy ancestors tried to improve the thermal insulation of their bodies when getting exposed to cold conditions, but also when there was a need to appear bigger in the face of adversaries. Cases have been reported of humans covered completely in hair, or possessing a little tail or having additional nipples. However, such cases are atavisms and have little to do with vestigial organs or rudimentary behaviour. They need to be interpreted as an unusual expression of ancestral traits dormant in the genome perhaps for hundreds or thousands of generations, but somehow activated during development and then showing up in some individuals. Atavisms are such an interesting phenomenon that I shall focus on that topic sometime in a future blog.

meyer rochow tails animal evolution

For more reading about the tails, click here for the “Tale of tails”

© Dr V.B. Meyer-Rochow and, 2017.
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